In vacuum moulding, the mould is usually consolidated by suction removal of the air present in the moulding sand with the mould surface being covered by a foil or the like and thus made airtight. To increase the air throughput during the suction removal step, suction pipes are generally provided in the flask and are embedded in the moulding sand. The suction pipes and the flask have suction surfaces in the moulding sand and these suction surfaces must, like screen or mesh surfaces, be formed with apertures smaller than the minimum grain size of the moulding sand.
In the rugged operating conditions of casting, the suction surfaces are stressed by high temperatures and severe mechanical loads. These surfaces, which often take the form of fine wire screens, therefore usually have special support structures, the aim of which is to ensure that the suction surfaces are not damaged which would permit the moulding sand to be sucked away resulting in undesirable penetration of the casting material into the flask.
Means for protecting vacuum lines and suction pipes in vacuum moulding flasks, of course, are known in the prior art. For example, German Utility model No. 8 029 438 discloses a vacuum moulding box wherein a perforate tube is covered with a fine-mesh wire fabric. The fabric is to a large extent covered by perforate metal strips which are secured externally above the wire fabric on the perforate tube so that the apertures in the strips register with the apertures in the tube. However, the fine wire fabric used for the screen surface has the major disadvantage of being very severely stressed when being woven. Moreover, the stressing conditions which occur during casting also tends to age the wire material very rapidly causing breaks at the bends. Consequently, apertures larger than the minimum grain size of the sand are created in the wire fabric, so that moulding sand is sucked away. The severe thermal aging experienced by the wire fabric cannot be prevented by the additional protection provided by the metal strips, even though a minor portion of the mechanical stressing can be kept away from the wire fabric.
German patent specification No. 3 123 363 also discloses an apparatus for the preparation of moulds with vacuum lines in the flask interior. To prevent damage to the flask, the vacuum lines are connected to a common vacuum collector duct on at least one wall of the flask. Disposed between such ducts and the vacuum lines is a screen facility in the form of two parallel screens, one of which closes the duct and the other the vacuum lines and the space between the screens is filled with moulding sand when the mould is prepared. The aim of this step is to prevent moulding material from entering the flask in the event of damage to the suctin pipes or vacuum lines. The arrangement disclosed in this patent also employs a delicate screen surface of the vacuum lines, for instance, in the form of fine wire fabric. Here too, therefore, aging of the known wire fabric is a serious disadvantage and the cost of obviating any risk of damage is very high. Although moulding can be facilitated by variability in the disposal of the vacuum line, the flask nevertheless becomes complicated. Also, the air resistance is increased considerably by the additional layer of sand, so that suction removal of air by the vacuum lines and stable vacuum throughout the flask cannot be guaranteed.